dillon1000/react

Retain minimal variable declarations in DCE

Currently DCE can remove variable declarations that are unused, ie where all 
control-flow paths to usage of the variable are overwritten by a reassignment. 
We then have to reconstruct the original variable declaration at the appropriate 
block scope during LeaveSSA, which is complex and can actually be incorrect in 
some cases. 

This PR updates to ensure that DCE will not remove the original variable 
declaration for any variable that is used (even in the case of always being 
reassigned before use). The main changes are: 

* DCE retains variable declarations, but if a variable declaration is always 
shadowed by reassignments then DCE will rewrite StoreLocal -> DeclareLocal so 
that it can DCE the unused initial value. 

* BuildHIR now has to change its handling for reassignment destructure 
instructions with nesting. Nesting uses a temporary which would appear as a 
declaration of a new variable, which is incompatible with other reassignments. 
See comments in the file. 

* LeaveSSA is quite a bit simpler now, since we never need to reconstruct a 
declaration.